Posture and musculoskeletal load describe the posture a product demands of its users and the physical strain that results from it over time. Unlike a use error, this load does not show up in a single use event but only over weeks and years.
Why time is the decisive factor
An unfavorable posture has no consequences in an individual case. It becomes problematic through repetition and duration. For this reason, this kind of load eludes classical usability evaluation, which looks at individual use events.
Typical triggers in the design are a working height that does not match body height, controls outside the comfortable reach zone, devices that are held continuously instead of being set down, and displays that force a bent head posture.
How load is assessed
Established observation methods exist for assessing working postures, such as RULA and REBA, which combine posture, force exertion, and repetition into a risk rating. They provide a traceable number instead of an impression.
In addition, measurement methods that capture movement and force directly are used, such as motion analysis or electromyography. These make it possible to demonstrate whether a design change actually reduces the load rather than merely shifting it.
Relevance in the clinical setting
In everyday clinical work, several stressors come together: patient transfers, prolonged standing, cramped spaces, and devices that are operated many times over the course of a shift.
This is relevant for manufacturers because the load on staff can be a selection criterion in purchasing. A device that forces a constrained posture is rated worse in a comparison even if its function is equivalent.
Typical weaknesses in practice
- Assessment of a single use event. The usability test looks at one individual use event. The load over an entire shift remains invisible.
- One working height for everyone. Fixed heights and reach zones without an adjustment range fit only some of the users.
- Impression instead of assessment. Postures are judged by eye, without a method such as RULA or REBA and without reference to a standard.
- Shifting instead of relieving. A design change relieves one joint and loads another. Without measurement, this goes unnoticed.
Regulatory reference
Two relevant standards exist for assessing body postures. ISO 11226 (edition 2000) contains recommended limits for static working postures and takes body angles and time fractions into account. DIN EN 1005-4 (edition 2009-01; European standard EN 1005-4) deals with the evaluation of working postures and movements in relation to machinery. Both stem from occupational and machinery ergonomics and are not medical device standards. They do, however, provide benchmarks against which a posture can be judged.
For medical devices, the MDR (EU Medical Device Regulation) requires in Annex I that risks resulting from the ergonomic features of the device be reduced as far as possible.
Musculoskeletal load does not arise in the individual use event but through repetition and duration, and it therefore eludes classical usability evaluation. It is assessed with observation methods such as RULA and REBA, complemented by motion analysis or electromyography. For manufacturers it matters because the load on staff can be a selection criterion in purchasing and because constrained postures promote use errors.
Frequently asked questions (FAQ)
How does musculoskeletal load differ from a use error?
A use error occurs in a single use event and is observable there. Musculoskeletal load arises through repetition and duration and shows itself only over longer periods. The two therefore require different survey methods.
Which methods are used for assessment?
Widely used are observation methods such as RULA and REBA, which combine posture, force exertion, and repetition into a risk rating. Motion analysis and electromyography are used in addition when the goal is to demonstrate whether a design change actually reduces the load.
Is musculoskeletal load relevant for medical device manufacturers?
Yes, for two reasons. First, the load on staff can be a selection criterion in purchasing. Second, a forced constrained posture can lead to errors in use and thus become safety-relevant as well.
Are there standards for assessing body postures?
Yes. ISO 11226 contains recommended limits for static working postures, and DIN EN 1005-4 deals with postures and movements when working with machinery. Neither is a medical device standard, but both provide benchmarks for judging a posture.
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